1. Field of the Invention
The present invention relates to a method and an apparatus for producing a thin film of a compound having a large area. More particularly, it relates to a method for producing a thin film of a compound on an elongate substrate or a large substrate by sputtering the compound on a continuously supplied substrate to form a thin film of the compound on the substrate, and an apparatus for carrying out said method.
2. Description of the Related Art
The superconductive phenomenon is said to be a phase transition of electrons and is a phenomenon in which a conductor has zero electric resistance and exhibits complete diamagnetism under specific conditions.
In electronic engineering, which is one of the technical fields in which the superconductive phenomenon can be utilized, various superconductive devices have been and are presently being proposed and developed. One of the typical devices is an element utilizing the Josephson effect in which the quantum effect is macroscopically exhibited by applied current. Since the superconductor has a small energy gap, a tunnel junction type Josephson element comprising the superconductor is expected to function as a high speed switching element with small power consumption. Further, since the Josephson effect appears as an exact quantum phenomenon against an electromagnetic wave or magnetic field, a Josephson element will be used as a highly sensitive sensor for sensing magnetic fields, microwaves, radiation, etc.
Since, in a very high speed computer, power consumption per unit area almost reaches the limit of cooling capacity, it is highly desired to provide superconductive elements. In addition, as the degree of integration in an electric circuit is increased, use of a superconductor as a wiring material is expected with little or no current loss.
In spite of enormous efforts, superconductor critical temperatures (Tc) did not exceed the 23K of Nb.sub.3 Ge for a long time.
In recent years, a sintered material of an oxide such as (La,Ba).sub.2 CuO.sub.4 or (La,Sr).sub.2 CuO.sub.4 was found to be a superconductor with high Tc and is expected to realize high temperature superconductivity. In such oxide superconductors, a Tc of 30 to 50K and even 70K or higher is observed.
In addition, a composite oxide of the formula: Y.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7-w, which is referred to as "YBCO", is reported to be a 90K superconductor. Further, Bi-Sr-Ca-Cu type and TI-Ba-Ca-Cu type composite oxides not only have Tc higher than 100K, but also are chemically stable so that their superconductive properties are less deteriorated as time passes, as compared to the superconductive properties of YBCO, which may deteriorate as time passes.
Since the oxide superconductor is prepared as a sintered material, it is usually brittle and requires great care in handling. That is, the ceramic oxide superconductor is easily broken or cracked by mechanical stress. Particularly, the ceramic oxide superconductor in the form of a wire is very easily broken. Therefore, its practical use is severely limited.
Furthermore, it is very difficult to form a sintered superconductor from homogeneous polycrystal consisting of particles all having superconductive characteristics. Moreover, as a general property of superconductors, the superconductive state may be locally broken by fluctuations in the external magnetic field or cooling temperature. The ceramic oxide superconductor has smaller thermal conductivity and larger electrical resistance than the classical superconductors. Therefore, when the superconductive state is locally broken, such parts of the superconductor are locally heated by the electric current which flows through the superconductor. If the cooling medium contacts a locally heated part of a superconductor, it is explosively vaporized.
To prevent such explosive vaporization, the classical metal superconductor is processed in the form of a thin filament and a plural number of filaments are bundled together by a good conductive material such as copper, which acts as a thermal conductor and a by-pass of electric current in case of loss of the superconductive state. However, it is difficult to process the ceramic oxide superconductor in the form of a filament produced by sintering and having high Tc.
To produce the wire or filament form oxide superconductor, it is essential to continuously form a thin film of the superconductor. As a result of extensive study, the present inventors have found that a sputtering method in which the superconductive material is used as a target is most suitable for forming a practically applicable thin film of the oxide superconductor.
However, since the oxide superconductor comprises several elements having different vapor pressures, adsorption probabilities and reaction rates, the composition of the compound formed on the substrate differs from that of the target, or the elementary composition on the surface of the target varies during sputtering.
When the thin film is formed on an elongate substrate or a large substrate, it takes a long time to form the thin film on the whole surface of the substrate. Therefore, the composition of the target varies during sputtering, when the compound itself is used as the target.